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    Auto-Bias board on Roy's website?

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    aguaazul

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    Re: Auto-Bias board on Roy's website?

    Post by aguaazul on Thu May 24, 2018 1:49 am

    Roy, Thanks for posting that! Now I can copy your expertise in my amp.  Good Vendor Salute!
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    Re: Auto-Bias board on Roy's website?

    Post by tubes4hifi on Thu May 24, 2018 11:41 am

    aquaazul
    I thought you already had that in you (DS) amp?
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    aguaazul

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    Re: Auto-Bias board on Roy's website?

    Post by aguaazul on Thu May 24, 2018 12:36 pm

    I have a DS2 - SP-14. My Latino ST-120 is on the bench waiting for a couple of spare hours to put it together, along with the Octal board.
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    deepee99

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    Re: Auto-Bias board on Roy's website?

    Post by deepee99 on Thu May 24, 2018 2:05 pm

    aguaazul wrote:I have a DS2 - SP-14. My Latino ST-120 is on the bench waiting for a couple of spare hours to put it together, along with the Octal board.
    I have Pavel's AB Board in my Sachs Kootenay, whose roots are in the VTA ST-120. Not one bit of grief with it. In fact, just the opposite. I like the slow B+ ramp-up -- slow but not too slow. I have entrusted my venerable Brit KT-88s to it. You do lose a bit of power, like a drop from 65 WPC to 60 WPC, but that's barely measureable in dB terms.
    IMHO, it should be an option for at least the VTA ST-120 kits. OTOH, that deviates somewhat from Robert's and Roy's KISS philosophy, which helped get me back into tubes in the first place.
    Maybe it's best to stick with the minimalist approach. Off-the-shelf VTAs and SPs will astound enough as-is. Then, down the road, start adding the bells and whistles.
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    pavlikkkk

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    Re: Auto-Bias board on Roy's website?

    Post by pavlikkkk on Sun May 27, 2018 6:55 am

    deepee99 wrote:
    aguaazul wrote:I have a DS2 - SP-14. My Latino ST-120 is on the bench waiting for a couple of spare hours to put it together, along with the Octal board.
    I have Pavel's AB Board in my Sachs Kootenay, whose roots are in the VTA ST-120. Not one bit of grief with it. In fact, just the opposite. I like the slow B+ ramp-up -- slow but not too slow. I have entrusted my venerable Brit KT-88s to it. You do lose a bit of power, like a drop from 65 WPC to 60 WPC, but that's barely measureable in dB terms.
    IMHO, it should be an option for at least the VTA ST-120 kits. OTOH, that deviates somewhat from Robert's and Roy's KISS philosophy, which helped get me back into tubes in the first place.
    Maybe it's best to stick with the minimalist approach. Off-the-shelf VTAs and SPs will astound enough as-is. Then, down the road, start adding the bells and whistles.

    When using AB-Q, it may happen that you measure slightly less output power than before. The explanation is simple. If the PP stage is not optimally set, the amplifier produces a larger third harmonic frequency. It is possible to measure higher output in the sum of the output signals. If the PP grade is exactly exported (AB-Q), the signal is "cleaned". The amount of harmonic frequencies decreases and the sum of the measured power is lower. A small loss of performance will bring great gain in signal quality.
    This phenomenon is typical especially in the M125 amplifier with quadruple end tubes where it is not possible to set the cathode current of the end tubes separately (pair cathodes are connected). Distorted signal on oscilloscope is not visible, the difference is not big. You'd have to measure power and distortion at the same time…
    Pavel
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    Kramer

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    Re: Auto-Bias board on Roy's website?

    Post by Kramer on Mon May 28, 2018 8:15 pm

    tubes4hifi wrote:seems no one has posted a photo of the auto-bias board in a working amp, so here's a photo of one I installed in a customer amp yesterday, with fuses on each output tube


    I just want to check and make 100% sure. The image posted looks like the AB cathode connection is going to pin 8 on both sockets and the fuse used to connect pin 8 to 1.

    The cathode connection should go to the unused pin 1 and the fuse should be used to bridge the connection to the cathode at pin 8.

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    pavlikkkk

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    Re: Auto-Bias board on Roy's website?

    Post by pavlikkkk on Wed May 30, 2018 1:41 am

    Kramer wrote:
    tubes4hifi wrote:seems no one has posted a photo of the auto-bias board in a working amp, so here's a photo of one I installed in a customer amp yesterday, with fuses on each output tube


    I just want to check and make 100% sure. The image posted looks like the AB cathode connection is going to pin 8 on both sockets and the fuse used to connect pin 8 to 1.

    The cathode connection should go to the unused pin 1 and the fuse should be used to bridge the connection to the cathode at pin 8.


    When using fuses, the reference voltage calculation of the AB-Q module changes!
    To set the correct reference voltage now apply: Vref = Icath x (Rcath+Rfuse).
    Rcath=10 Ohm
    Rfuse=1,5 Ohm
    Icath=55 mA
    For example set Icath current 55 mA:
    Reference voltage: .055 * 11.5 = .63V

    Pavel


    Last edited by tubes4hifi on Sat Jun 02, 2018 8:55 pm; edited 1 time in total (Reason for editing : incorrect equation, was divide instead of multiply)
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    pichacker

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    Re: Auto-Bias board on Roy's website?

    Post by pichacker on Wed May 30, 2018 2:31 am

    Pavel,

    Why should the reference calculation change because of the fuse? If the fuse is connected between the cathode and the autobias board then it is not in the feedback loop of the cathode resistor. The bias sampling voltage still comes from across the 10R resistor so should not change.

    Cathode ---> Fuse ---> X 10R X ---> Gnd.

    https://photos.app.goo.gl/bhA0jyiIDbOEr0n93

    If the bias current is altered by measuring the voltage across the 10R (X above) then the fuse will not make a noticable difference surely?

    On a different note regarding the removal of the power on delay and solid state rectification. If the bias board holds the output valves biased off until they have warmed up, then with no power on delay, solid state rectification, and no load, the main resovoir cap will be charge to peak voltage until normal operation is established. As I have yet to receive my board (still in the post) I have not made any measurements.

    I also note that there is a terminal for "HiV" on the auto bias board. Looking at the instructions it appears that this could be connected to the HT line to control the operation of the bias board and start the power on delay process once HT has been established? Am I reading this correctly? If this is the case then what are the voltage ranges allowable on this pin please?

    Just being a devils advocate but I note the mini fuses are rated for 250V. If an output tube suffers a breakdown or flashover from anode to cathode then in thoery full HT (minus drop across opt) could be briefly applied to the fuse.

    Steve
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    pavlikkkk

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    Re: Auto-Bias board on Roy's website?

    Post by pavlikkkk on Wed May 30, 2018 11:24 am

    pichacker wrote:Pavel,

    Why should the reference calculation change because of the fuse? If the fuse is connected between the cathode and the autobias board then it is not in the feedback loop of the cathode resistor. The bias sampling voltage still comes from across the 10R resistor so should not change.

    Cathode ---> Fuse ---> X 10R X ---> Gnd.

    https://photos.app.goo.gl/bhA0jyiIDbOEr0n93

    If the bias current is altered by measuring the voltage across the 10R (X above) then the fuse will not make a noticable difference surely?

    On a different note regarding the removal of the power on delay and solid state rectification. If the bias board holds the output valves biased off until they have warmed up, then with no power on delay, solid state rectification, and no load, the main resovoir cap will be charge to peak voltage until normal operation is established. As I have yet to receive my board (still in the post) I have not made any measurements.

    I also note that there is a terminal for "HiV" on the auto bias board. Looking at the instructions it appears that this could be connected to the HT line to control the operation of the bias board and start the power on delay process once HT has been established? Am I reading this correctly? If this is the case then what are the voltage ranges allowable on this pin please?

    Just being a devils advocate but I note the mini fuses are rated for 250V. If an output tube suffers a breakdown or flashover from anode to cathode then in thoery full HT (minus drop across opt) could be briefly applied to the fuse.

    Steve

    You're right. If the cathode resistors remain on the module (the original), then nothing has to be recalculated  is true Uref = Icath x 10 (I measured it).
    If you dismantle the resistors, add the fuses and mount them to the tube plinths, connect the AB-Q CATH input to the cathodes. Then you have to recalculate it Uref.
    Further,
    After switching on the amplifier (without anode voltage delay) grids Tubesand have the highest negative voltage from the bias source.
    After about 90 sec. tubes are gradually opening.
    It is not necessary to connect the HiV to anode voltage delays.

    Remark the fuse voltage rating is not correct, but I think the fuses fulfills the function even if the 250V voltage is exceeded.

    The maximum allowed voltage at HiV is 400V


    Pavel
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    pichacker

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    Re: Auto-Bias board on Roy's website?

    Post by pichacker on Thu May 31, 2018 3:02 am

    Pavel,

    Thank you for the prompt response. Looking forward to receiving my board soon.

    If anyone lifted the resistor on the board and added the fuse there this would be counter productive to the initial task of attempting to protect the bias board from a high voltage. The fuse on the valve base is probably the best place in my opinion.

    I was wondering if it would be advantageous to add a "sacrificial" zener between the cathode connection to the bias board and ground (possibly at the tube base). Under normal circumstances the zener would not conduct but under fault conditions it would prevent the higher than normal voltage spike entering the board, clamping the voltage for long enough for the fuse to blow.

    https://photos.app.goo.gl/ozWaSeRoNkj6D7GN2

    Cheers

    Steve


    Last edited by pichacker on Thu May 31, 2018 4:47 am; edited 1 time in total
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    Re: Auto-Bias board on Roy's website?

    Post by pavlikkkk on Thu May 31, 2018 4:09 am

    pichacker wrote:Pavel,

    Thank you for the prompt response. Looking forward to receiving my board soon.

    If anyone lifted the resistor on the board and added the fuse there this would be counter productive to the initial task of attempting to protect the bias board from a high voltage. The fuse on the valve base is probably the best place in my opinion.

    I was wondering if it would be advantageous to add a "scarificial" zener between the cathode connection to the bias board and ground (possibly at the tube base). Under normal circumstances the zener would not conduct but under fault conditions it would prevent the higher than normal voltage spike entering the board, clamping the voltage for long enough for the fuse to blow.

    https://photos.app.goo.gl/ozWaSeRoNkj6D7GN2

    Cheers

    Steve


    there must be a resistor. It evaluates the voltage change for comparison with reference set to AB-Q.
    Cheers
    Pavel
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    pichacker

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    Re: Auto-Bias board on Roy's website?

    Post by pichacker on Thu May 31, 2018 4:56 am

    Pavel,

    Wasn't suggesting removing the resistor, I appreciate this is needed for the system to function.
    Just thought that the best place would be on the board as you have designed it.

    If the zener voltage is at a level much greater than the nominal voltage developed by the standing bias, or music peaks, then it will sit there doing nothing until a fault occurs. I'd rather change an off board fuse and possibly a zener than an op amp on a pcb.

    Cheers

    Steve
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    Re: Auto-Bias board on Roy's website?

    Post by pavlikkkk on Thu May 31, 2018 7:26 am

    pichacker wrote:Pavel,

    Wasn't suggesting removing the resistor, I appreciate this is needed for the system to function.
    Just thought that the best place would be on the board as you have designed it.

    If the zener voltage is at a level much greater than the nominal voltage developed by the standing bias, or music peaks, then it will sit there doing nothing until a fault occurs. I'd rather change an off board fuse and possibly a zener than an op amp on a pcb.

    Cheers

    Steve
    I see.., you mean the zener parallel to the resistor,
    I try it on the tester and write it.
    Pavel
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    Re: Auto-Bias board on Roy's website?

    Post by pavlikkkk on Thu May 31, 2018 3:55 pm

    pichacker wrote:Pavel,

    Thank you for the prompt response. Looking forward to receiving my board soon.

    If anyone lifted the resistor on the board and added the fuse there this would be counter productive to the initial task of attempting to protect the bias board from a high voltage. The fuse on the valve base is probably the best place in my opinion.

    I was wondering if it would be advantageous to add a "sacrificial" zener between the cathode connection to the bias board and ground (possibly at the tube base). Under normal circumstances the zener would not conduct but under fault conditions it would prevent the higher than normal voltage spike entering the board, clamping the voltage for long enough for the fuse to blow.

    https://photos.app.goo.gl/ozWaSeRoNkj6D7GN2

    Cheers

    Steve

    Probably the best thing would be to use a varistor, for example LITTELFUSE V8ZA1P.
    It would not have to change after the short circuit.


    Pavel


    I tried it, it works perfectly.


    Last edited by pavlikkkk on Wed Jun 06, 2018 4:56 am; edited 1 time in total
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    tubes4hifi
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    Re: Auto-Bias board on Roy's website?

    Post by tubes4hifi on Sat Jun 02, 2018 8:59 pm

    to Kramer, and others regarding . . . The image posted looks like the AB cathode connection is going to pin 8 on both sockets and the fuse used to connect pin 8 to 1.

    The cathode connection should go to the unused pin 1 and the fuse should be used to bridge the connection to the cathode at pin 8.


    On all octal output power tubes normally used such as EL34, KT88, etc, the cathode is pin 8, not pin 1. In some tubes pin 1 is not connected, in other tubes it is connected to the suppressor grid,
    in some tubes pin 1 and 8 are internally connected. So the cathode connection is actually pin 8. In most all amps pin 1 and pin 8 are connected externally.
    Now my first thought would be to connect the fuse from pin 8 to ground, but my associate and long-time tube tech Don Sachs has recommended this connection
    from pin 1 (suppressor grid) to pin 8 (cathode) to protect against INTERNAL tube shorts caused by overcurrent/internal overheating.
    Also, it should be obvious that if the fuse is only rated at 250v then that is NOT a problem, because the cathode and suppressor should never have more than about -60vdc on them.
    And if they do go above 250v, why is that a problem? The fuse should blow!!

    Also, as for the fuses I am using/recommending, they are only 1/4 ohm, far less than most other similar fuses, this changes the bias reference voltage by only 2.5% rather than 15%, not enough to bother with

    Also, for those who don't seem to read the instructions (or maybe don't understand them) the HV connection on the auto-bias board is normally NOT used,
    it would only be used if you were delaying the B+ turn-on by MORE THAN ONE FULL MINUTE.
    If you have a TDR installed, the delay is only 40 seconds max, so the HV connection on the AB board is NOT used !!
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    Re: Auto-Bias board on Roy's website?

    Post by tubes4hifi on Sat Jun 02, 2018 9:09 pm

    here's a photo of my third auto-bias install in an ST120 amp, in this photo I haven't put fuses in yet, but this is a cleaner amp build than the photo I posted a couple weeks ago.
    About 45 minutes this time rather than 90 minutes on the first one I did, practice makes perfect and cuts time!  Also the old 10 ohm cathode resistors are just cut off and not connected on one end.
    12 EZ wire connections, and pop out the old bias pots and R37-R38 and that's it, simple.

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    Re: Auto-Bias board on Roy's website?

    Post by tubes4hifi on Sat Jun 02, 2018 9:55 pm

    here's a photo of it working, 456vdc, this ST120 amp has the TDR which was set for around 20 seconds, and bias is at 55ma per output tube,
    This amp is using a rectifier tube, so that rectifier is running at MAX current spec, highly suggest a WZ68 rectifier.



    Last edited by tubes4hifi on Sun Jun 03, 2018 4:38 pm; edited 3 times in total (Reason for editing : updated photo)
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    Kramer

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    Re: Auto-Bias board on Roy's website?

    Post by Kramer on Sun Jun 03, 2018 2:09 pm

    Roy, thanks for clearing that up and explaining the method in more detail. Very nice wiring on that latest board install.
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    Re: Auto-Bias board on Roy's website?

    Post by pichacker on Mon Jun 04, 2018 2:49 am

    Pavel,

    I like the suggestion regarding using a MOV. Only the fuse and a tube to change in the event of a flashover.

    My board got delivered on Friday and whilst looking over it, and sketching out the circuit, I noticed that the cathode end of the 10R resistor is very close to an adjacent track.

    https://photos.app.goo.gl/gv2txwwmSw9EQJVw9

    I am definately going the MOV/Fuse route when I install the board to lessen the chances of a flashover to this track, also if the Zener method was used and the Zener shorted then of course this would remove any feedback monitoring of the cathode current. Result would be the bias board would turn the tube hard on!

    I'll post some more photos as I go on.

    Roy,

    looking at a combination of instructions and photos I can see in one of them that there are dual ground connections. One from the gnd point next to the HiV to the common star ground and then another from the bias connections. I can see on your later photos that the common ground point one is not used. (Posts 89 schematic and photo post 166)

    Since we do not want to create a loop, which ground connection do you find best to use. To the star point or from the bias supply? I'm leaning towards taking the ground from the CCS driver board bias supply.

    Also have you noted any hum introduction when one side of the heater windings is grounded?

    Cheers

    Steve
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    Re: Auto-Bias board on Roy's website?

    Post by tubes4hifi on Mon Jun 04, 2018 12:11 pm

    PicHacker,
    there is only one ground connection to the board, the HV and GND connections are NOT used.
    Ground is from the driver board ground or can be from the amp chassis ground, either works since they are the same other than about 3" of wire.
    I haven't found any hum added with the filament connection.
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    pichacker

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    Re: Auto-Bias board on Roy's website?

    Post by pichacker on Tue Jun 05, 2018 4:29 pm

    Fitted the board last night along with the cathode fuse/mov protection circuits. It works a treat Smile

    It felt a bit like performing open heart surgery on an old friend as I had to de-solder and lift the driver board out to remove the pots and lay down the bias caps to avoid fouling. Also for us over here in Europe I had to remove the imperial screws and replace them with metric ones to allow me to screw on the 4 spacers for the bias board to mount to.

    Connecting up went without any issues and I used a single ground wire to the common point on the driver board bias supply. For the filament connections I used the side nearest to the input terminal which also happened to be the side that does not feed the centre tube. This allows for the centre tube to still have a floating filament supply.

    For my cathode protection circuits I decided to mount the components on a three lug piece of tag board secured under a mounting screw. One for each output tube. That way any local fault currents will have the shortest way back to chassis and also allowed the output tubes to still have the pins linked as original. No real need to do this but just my preference.



    (Must get around to uprating the 470R on the main cap as it runs a little too hot for my liking)

    Once set up, the tube bias current is rock steady. I have yet to put it through its paces with extremes of line voltages or changing from SS to tube rectification but I have no doubt that it will cope without any issues. I have sketched out the circuit and it is very well designed. I was actually surprised at the length of the time constant used in the bias regulator.

    I kept my HT switch on delay circuit still in operation and it applies HT at about the same time as the red LED on the bias board extinguishes. There are still many seconds after this event before the tubes ramp up to normal operating current. No perceived issues.

    I attempted power cycling to see what would happen, there were no issues here either, as the bias board reset relatively quickly and always ramped the current up smoothly. I would not recommend trying this with a tube rectifier installed.

    Well done Pavel, a nice design which appears very stable with about the right time constants and gain in the control loop.



    Amp is dead quite with no hum from the speakers.  cheers


    Incidentally as I don't use the front mounted sockets in my amp I connected the cathode leads to the monitor points on the driver board. That way I can connect my fluke to the topside when the amp is together and measure the bias voltage to confirm things are still in order.
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    aguaazul

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    Re: Auto-Bias board on Roy's website?

    Post by aguaazul on Wed Jun 06, 2018 1:15 pm

    aguaazul wrote:I have a DS2 - SP-14. My Latino ST-120 is on the bench waiting for a couple of spare hours to put it together, along with the Octal board.

    Got it all wired up this morning. Using fuse holders for easy replacement.



    Will test tonight.

    Aguaazul

    Hosted picture is kind of huge. Here is a link to a google photos album.


    AB Board - Fuses w/ easy fuse change out
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    Re: Auto-Bias board on Roy's website?

    Post by aguaazul on Thu Jun 07, 2018 9:17 am

    pavlikkkk wrote:
    pichacker wrote:Pavel,

    Thank you for the prompt response. Looking forward to receiving my board soon.

    If anyone lifted the resistor on the board and added the fuse there this would be counter productive to the initial task of attempting to protect the bias board from a high voltage. The fuse on the valve base is probably the best place in my opinion.

    I was wondering if it would be advantageous to add a "sacrificial" zener between the cathode connection to the bias board and ground (possibly at the tube base). Under normal circumstances the zener would not conduct but under fault conditions it would prevent the higher than normal voltage spike entering the board, clamping the voltage for long enough for the fuse to blow.

    https://photos.app.goo.gl/ozWaSeRoNkj6D7GN2

    Cheers

    Steve

    Probably the best thing would be to use a varistor, for example LITTELFUSE V8ZA1P.
    It would not have to change after the short circuit.


    Pavel


    I tried it, it works perfectly.

    Pavel,

    So add the MOV between pin 8 & ground?

    Aguaazul
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    Re: Auto-Bias board on Roy's website?

    Post by tubes4hifi on Thu Jun 07, 2018 2:37 pm

    yes, pin 8 is the cathode
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    Re: Auto-Bias board on Roy's website?

    Post by pichacker on Fri Jun 08, 2018 6:29 am

    If pin 8 is the cathode and pin 1 unused then the MOV should go the other side of the fuse. Ie pin 1 to ground.

    [Pin 8 Cathode] -- [FUSE] -- [Pin 1] -- [Bias Board and MOV to ground]

    That way the MOV clamps the input to the bias board and the fuse then fails.


    Roy, In post 167 the lead from the bias board appears to be connected to pin 8 of the tube? If that is the cathode connection then the fuse is not in circuit.


    Last edited by pichacker on Fri Jun 08, 2018 7:04 am; edited 1 time in total

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